Group III-V compound semiconductors comprising boron (B) which belongs to Group III of the Periodic Table, and an element which belongs to Group V include boron nitride (BN), boron phosphide (BP), and boron arsenide (BAs). For example, hexagonal boron nitride (BN) is an indirect transition-type semiconductor having a band gap of 7.5 electron volts (eV) at room temperature (refer to Iwao Teramoto, Handotai Debaisu Gairon (Outline of semiconductor Devices), 1st Ed., page 28, Baifukan (Mar. 30, 1995)). Boron arsenide (BAs) is known to be an indirect transition-type Group III-V compound having a band gap of about 0.85 eV at room temperature (refer to Handotai Debaisu Gairon (Outline of semiconductor Devices)).
On the other hand, boron phosphide (BP) is a kind of Group III-V compound semiconductor (refer to Nature, 179, No. 4569, page 1075 (1957)) and several values are reported for the band gap of an indirect transition-type semiconductor. For example, B. Stone et al. reports a room temperature band gap of about 6 eV for a polycrystalline BP layer deposited on a quartz plate, which is determined by an optical absorption method (refer to Phys. Rev. Lett., Vol. 4, No. 6, pages 282 to 284 (1960)). According to J. L. Peret, the band gap of BP is determined to be 6.0 eV (refer to J. Am. Ceramic Soc., 47 (1), pages 44 to 46 (1964)). Also, N. Sclar reports, that based on the ion radius value and the covalent radius value, the band gap at absolute zero (=0 K) is 6.20 eV (refer to J. Appl. Phys., 33 (10), pages 2999 to 3002 (1962)). Furthermore, Manca reports a band gap of 4.2 eV (refer to J. Phys. Chem. Solids, 20, pages 268 to 273 (1961)).
On the other hand, R. J. Archer et al. reports a room temperature band gap of 2 eV for cubic BP, which is determined from a single crystal BP grown from a nickel phosphide fused solution (refer to Phys. Rev. Lett., 12 (19), pages 538 to 540 (1964)), and also reports a band gap of 2.1 eV which is determined from a theoretical calculation based on the bonding energy value (refer to J. Appl. Phys., 36, pages 330 to 331 (1965)). Thus, the band gap of boron phosphide (BP) greatly differs (refer to J. Phys. Chem. Solids, 29, pages 1025 to 1032 (1968)), however, a value of about 2 eV has been heretofore commonly employed for the band gap of BP (refer to (1) RCA Review, 25, pages 159 to 167 (1964); (2) Z. anorg. allg. chem., 349, pages 151 to 157 (1967); (3) J. Appl. Phys., 36 (1965); (4) Handotai Debaisu Gairon (Outline of Semiconductor Devices) page 28; and (5) Isamu Akasaki, III-V Zoku Kagobutsu Handotai (Group 111-V Compound Semiconductors), 1st Ed., page 150, Baifukan (May 20, 1994)).
The boron phosphide (BP) and the BP-base mixed crystal represented by the compositional formula of BXAlYGa1-X-YN1-ZPZ (wherein 0<X≦1, 0≦Y<1, 0<X+Y≦1, 0<Z≦1) are used as a functional layer constituting a semiconductor light-emitting device. In conventional techniques, for example, a single layer comprising BP is used for constituting a buffer layer in a short wavelength visible light-emitting diode (LED) or laser diode (LD) (refer to, JP-A-2-275682) (the term “JP-A” as used herein means an “Japanese Unexamined Patent Application, First Publication No.”). Also, a case is known where the light-emitting part of a pn junction-type heterojunction structure is constructed by a super lattice structure of a BP single layer with a BXAlYGa1-X-YN1-ZPZ mixed crystal single layer (refer to JP-A-10-242514). Furthermore, a technique of constructing a clad (barrier) layer by a super lattice structure with a BXAlYGa1-X-YN1-ZPZ mixed crystal single layer is known (refer to JP-A-2-288371). Since boron phosphide (BP) having a band gap of 2 eV at room temperature cannot exert a barrier effect on the light-emitting layer, in the above-described conventional case, a nitrogen-containing mixed crystal layer elevated in the room temperature band gap, for example, to 2.7 eV by forming BP and aluminum nitride (AlN) or the like into a mixed crystal is used (refer to JP-A-2-288371).
Also, a case where a hetero bipolar transistor (HBT) is constructed by using a BP single layer is known (refer to J. Electrochem. Soc., 125(4), pages 633 to 637 (1978)). In this conventional HBT, a BP single layer having a band gap of 2.0 eV is used, which is grown on a silicon (Si) crystal substrate having a (100) plane by a diborane (B2H6)/phosphine (PH3) vapor phase growth method (refer to, J. Electrochem. Soc., 125 (1978)). Furthermore, a technique of constructing a solar cell by using a BP single layer having a band gap of 2.0 eV as a window layer is disclosed (refer to J. Electrochem. Soc., 125 (1978)).
As described above, semiconductor devices have been heretofore constructed by using boron phosphide (BP) having a band gap of about 2 eV or a mixed crystal containing BP having this band gap. In the above-described solar cell constructed by using Si having a room temperature band gap of 1.1 eV as the matrix material, it is disclosed that even a BP layer having a band gap of 2.0 eV can be effectively used as the window layer because the band gap is larger than that of the matrix Si (refer to, J. Electrochem. Soc., 125 (1978)). On the other hand, however, in conventional techniques of forming a BP layer using Si as the substrate, it is reported that the band gap depends on the plane direction of an Si single crystal as the substrate and is narrowed (refer to Tatau Nishinaga, Oyo Butsuri (Applied Physics), Vol. 45, No. 9, pages 891 to 897 (1976)). Also, it is reported that as compared with the BP layer formed on an Si substrate having a (100) plane, the BP layer formed on an Si substrate having a (111) plane has a large plane defect density and therefore, becomes opaque (refer to Oyo Butsuri (Applied Physics), pages 895 to 896).
Furthermore, it is reported that the lattice constant becomes large due to a large amount of plane defects and the band gap is narrowed even more (refer to Oyo Butsuri (Applied Physics), page 896). The lattice constant and the band gap are conventionally known to be correlated with each other and, as is well known, the band gap increases as the lattice constant becomes smaller (refer to III-V Zoku Kagobutsu Handotai (Group III-V Compound Semiconductors), page 31). In other words, according to conventional studies, it is taught that a BP layer having a band gap smaller than about 2 eV, which is commonly employed as the band gap of BP, is formed depending on the conditions of forming the BP layer. This small band gap gives rise to a problem in that an environment-resistant semiconductor device having a high breakdown voltage cannot be readily constructed from the boron phosphide (BP) crystal layer.
For example, in a heterojunction-type blue LED or LD having a light emission wavelength of 450 nm at room temperature, a light-emitting layer having a room temperature band gap of 2.8 eV is used. In order to achieve a clad effect on this light-emitting layer, the barrier layer must be composed of a semiconductor material having a room temperature band gap of at least about 2.8 eV. Therefore, in constructing a heterojunction light-emitting part of a conventional boron phosphide (BP)-based light-emitting device, there is a problem in that the clad layer cannot be composed of boron phosphide (BP) having a room temperature band gap of about 2 eV. To overcome this problem, in conventional techniques, a mixed crystal containing BP, for example, a BXAlYGa1-X-YN1-X-ZPZ multi-element mixed crystal, is formed as described above and a barrier layer having a high band gap is constructed therefrom (refer to JP-A-2-288371). However, it is well known that as the number of elements constituting the mixed crystal increases, a higher level technique is required, for example, to control the composition ratio of the constituent elements and, moreover, a crystal layer having good quality is more difficult to obtain (refer to Handotai Debaisu Gairon (Outline of Semiconductor Devices), page 24). Thus, in view of layer formation, conventional techniques have a problem in that a BP mixed crystal layer as a barrier layer cannot be readily and simply formed.
Also, for example, in a conventional npn-type HBT, a BP layer having a band gap of 2.0 eV is used as an n-type emitter (refer to J. Electrochem. Soc., 125 (1978)). On the other hand, for the p-type base layer, a p-type Si layer is used (refer to J. Electrochem. Soc., 125 (1978)). The band gap of Si is about 1.1 eV and, therefore, the difference in the band gap between the heterojunction structures of the BP emitter layer and the Si base layer is only 0.9 eV. It is considered that if the emitter layer is constructed by a BP layer, which gives a larger difference in the band gap between the emitter layer and the base layer than that in conventional techniques, the leakage of the base current from the base layer to the emitter layer can be suppressed more and the current transmission ratio (=emitter current/collector current) property can be improved (refer to Isamu Akasaki (editor) III-V Zoku Kagobutsu Handotai (Group III-V Compound Semiconductors), pages 239 to 242); as a result, an HBT having excellent characteristics can be produced.
The lattice constant of a boron phosphide (BP) single crystal which is a zinc blende crystal type, more accurately cubic sphalerite type, is 0.4538 nm (refer to Handotai Debaisu Gairon (Outline of semiconductor Devices), page 28). On the other hand, Group III nitride semiconductors having a lattice constant of 0.4538 nm are known, such as a cubic gallium phosphide nitride mixed crystal (compositional formula: GaN0.97P00.03) having a nitrogen (N) composition ratio of 0.97, and gallium indium nitride (Ga0.90In0.10N) having an indium (In) composition ratio of 0.10. Accordingly, if the BP layer and such a Group III nitride semiconductor are used, a two-dimensional electron gas field effect transistor (TEGFET) of a lattice-matching stacked layer type which is advantageous for obtaining high electron mobility can be constructed (refer to K. Seeger, Semikondakuta no Butsurigaku, (Ge) (Physics of Semiconductors (Final Volume)), 1st printing, pages 352 to 353, Yoshioka Shoten (Jun. 25, 1991)). For example, the TEGFET can be constructed by using the above-described direct transition-type Group III nitride semiconductor as a two-dimensional electron gas (TEG) channel layer and the indirect transition-type BP layer as a spacer layer or an electron supply layer. In the Group III nitride semiconductor TEGFET using a BP layer, if a spacer layer or an electron supply layer forming a heterojunction with an electron channel layer is composed of boron phosphide (BP) having a band gap larger than that in conventional techniques, the barrier difference at the heterojunction interface with the electron channel layer can be made large. This is advantageous for accumulating two-dimensional electrons in the region of the electron channel layer near the heterojunction interface. As a result, a Group III nitride semiconductor TEGFET realizing high electron mobility can be obtained.
If a BP layer having a large room temperature band gap can be used, the discontinuity of the conduction band with other semiconductor layers can also be made larger. A heterojunction structure having a large band discontinuity and a large barrier difference is effective for achieving high electron mobility, because two-dimensional electrons can be efficiently accumulated. In the case of a Hall device, which is a magnetoelectric converting device, use of a structure having high electron mobility is advantageous for obtaining a device having a higher sensitivity to magnetism (refer to Shoei Kataoka, Jiden Henkan Soshi (Magnetoelectric Converting Devices), 4th printing, pages 56 to 58, Nikkan Kogyo Shinbun (Feb. 1, 1971)). Accordingly, the realization of a heterojunction structure containing a BP layer having a band gap larger than that in conventional techniques is considered to contribute also to the construction of a high-sensitivity Hall device capable of exhibiting high product sensitivity (refer to Jiden Henkan Soshi (Magnetoelectric Converting Devices), page 56).
Also, for example, in the case of a Schottky barrier diode using an Si single crystal substrate, if a BP layer having a room temperature band gap in excess of about 2 eV can be formed, this can contribute to the construction of a Schottky barrier diode having a high breakdown voltage. It is considered that as the band gap is made larger, the intrinsic carrier density in the properties of a semiconductor material can be more suppressed (refer to (III-V Zoku Kagobutsu Handotai (Group III-V Compound Semiconductors), pages 172 to 174), and therefore, such a BP layer is advantageous for constructing an environment-resistant device capable of operating at high temperatures.
As in these conventional cases, semiconductor devices have been heretofore constructed by using a BP layer having a band gap of about 2 eV. If a BP layer having a larger band gap can be formed, it is expected that the semiconductor device can be improved and enhanced in its properties. In the studies so far, a case where a BP layer having a high band gap of about 6 eV is formed is known as described above (refer to Phys. Rev. Lett., 4 (6) (1960)). However, this is a polycrystalline layer and is not necessarily suitable for the construction of an active layer or a functional layer of a semiconductor device. In a wide gap semiconductor having such a large band gap, the control of the conduction type by impurity doping and the control of carrier density are difficult. A BP layer suitable for the construction of a functional layer in a semiconductor device, such as a spacer layer or an electron supply layer in a TEGFET or an emitter layer in an HBT, is a BP crystal layer having a band gap of about 3 eV.
According to studies so far on the band gaps of compound semiconductors, it is known that as the average atomic number of the constituent elements is made smaller, the band gap tends to become larger (refer to Kazuo Fueki, et al., Oyo Kagaku Shirizu 3, Denshi Zairyo no Kagaku (Applied Chemistry Series 3, Chemistry of Electronic Materials), pages 26 to 29, Maruzen (Jul. 20, 1981)). The average atomic number is the arithmetic mean value of the atomic numbers of the elements constituting the compound semiconductor. FIG. 1 shows the relationship between the band gap at room temperature and the average atomic number of various Group III-V compound semiconductors. For example, the room temperature band gap of gallium arsenide (GaAs) (average atomic number=32) comprising gallium (Ga) (atomic number=31) and arsenic (As) (atomic number=33) is 1.43 eV (refer to Handotai Debaisu Gairon (Outline of semiconductor Devices), page 28). On the other hand, the room temperature band gap of gallium phosphide (GaP) (average atomic number=23) having an average atomic number smaller than that of GaAs is as large as 2.26 eV (refer to Handotai Debaisu Gairon (Outline of semiconductor Devices), page 28). This relationship applies to Group II-VI compound semiconductors and it is taught that as the average atomic number of the constituent atoms is made smaller, the band gap tends to become larger (refer to K. Seeger, Semikondakuta no Butsurigaku (Jo) (Physics of Semiconductors (First Volume)), 1st printing, page 36, Yoshioka Shoten (Jun. 10, 1991)).
It is said that based on this tendency of the room temperature band gap in relation to the average atomic number, it can be presumed that the band gap of Group III-V compound semiconductors has a relatively large ionic bonding property. Assuming that this tendency is applicable also to a BP crystal having a small difference in the electronegativity between the constituent elements and having a strong covalent bonding property, the band gap of the BP single crystal layer is presumed to be about 3 eV: Furthermore, according to the “Dielectric Method” proposed by Van Vechten (refer to: (1) J. A. Van Vechten, Phys. Rev. Lett., 182 (1969), 891; and (2) Isamu Akasaki (editor), III Zoku Chikkabutsu Handotai (Group III Nitride Semiconductors), 1st Ed., pages 19 to 21, Baifukan (Dec. 8, 1999)), the band gap of a BP single crystal is calculated as 2.98 eV. In this theoretical calculation of the band gap, the lattice constants of carbon (diamond) (C) and silicon (Si) single crystals are set to 0.3567 nm and 0.4531 nm, respectively. The minimum interatomic distances of C (diamond) and Si are set to 0.154 nm and 0.234 nm, respectively (refer to Kagaku Binran Kisohen (Handbook of Chemistry, Elementary), 3rd printing, page 1259, Maruzen (Aug. 20, 1970)). As for the other values necessary for the calculation, the proposed values are used (refer to III Zoku Chikkabutsu Handotai (Group III Nitride Semiconductors), pages 20 to 21).
At present, boron phosphide (BP) having a room temperature band gap of about 3 eV and a boron phosphide (BP)-base mixed crystal containing a BP crystal thereof, of which a single layer is advantageous for the construction of the above-described semiconductor devices, have not yet been disclosed. This is attributable to the fact that a method for forming a BP crystal layer having excellent crystallinity is not clearly known. More specifically, a method for forming a layer which gives a BP-base mixed crystal having a band gap suitable for the construction of a semiconductor device is not clearly known. In order to improve the properties of semiconductor devices using a BP crystal layer, a method for forming a BP crystal layer having a band gap of about 3 eV must be established. However, despite the layer formation of BP crystal layers heretofore practiced using a vapor-phase growth method or the like, a method for forming a BP crystal layer which gives a band gap of about 3 eV has not yet been disclosed.